1. Field of the Invention
The invention relates to a fluorination process for producing fluorinated compounds which can be used in particular as electrolyte.
2. Description of the Related Art
Lithium batteries, in which the anode is formed by a sheet of lithium metal or by a lithium alloy and which operate by movement of lithium ions between the electrodes, have been widely studied. However, their development has been impeded due to the fact that, during their recharging, deposition of lithium metal of dendritic nature occurs, which can lead to short-circuits, resulting in an explosion in the system. This risk has been eliminated by replacing the lithium or lithium alloy anode by an anode composed of a carbonaceous material in which the lithium ions can be reversibly inserted. This novel form of lithium batteries, known as “lithium-ion” batteries, is widely used in the field of portable electronic equipment. The electrolyte of these batteries comprises at least one lithium salt in solution in an organic solvent which can be a polar aprotic liquid solvent (for example, ethylene carbonate, propylene carbonate or a dialkyl carbonate) optionally supported by a porous plastic support, a polar polymer [for example, a crosslinked poly(ethylene oxide)] or a liquid solvent gelled by a polymer. The lithium salt plays an important role in the operation of the battery. The most widely used salt is LiPF6, which makes it possible to obtain liquid electrolytes which have a conductivity of greater than 10−2 S.cm−1 at ambient temperature. However, it has a limited thermal stability, which results in the formation of LiF and of HF, said HF leading to decomposition of the electrolyte which can result in an explosion in the battery. The lithium salt of bis(trifluoromethanesulfonyl)imide has been envisaged for replacing LiPF6, but it exhibits the disadvantage of resulting in depassivation of the aluminum current collector of the cathode.
The use of imide salts or methane salts having FSO2 or F2PO electron-withdrawing groups was then studied (WO 95/26056). These salts make it possible to obtain electrolytes with a greater conductivity than their homologues comprising perfluoroalkyl groups instead of the fluorine atoms and they result in markedly lower corrosion of the aluminum collectors. The use of an imide salt or methane salt comprising FSO2 or F2PO groups thus makes it possible to maintain the low level of corrosion observed with LiPF6 while improving the thermal stability with respect to that of LiPF6.
Various processes for the preparation of imide salts or methane salts comprising at least one FSO2 or F2PO group have been described. For example, bis(fluorosulfonyl)imide (FSO2)2NH can be prepared by reaction of fluorosulfonic acid FSO3H with urea H2NC(O)NH2. The imide is subsequently isolated by treatment of the reaction mixture with NaCl in dichloromethane, followed by distillation of the pure acid [Appel & Eisenhauer, Chem. Ber., 95, 246–8, 1962]. However, the toxicity and the corrosive nature of FSO3H constitute a major disadvantage.
Another process consists in reacting (ClSO2)2NH with AsF3. The acid (FSO2)2NH is subsequently isolated by treating the reaction mixture with NaCl in dichloromethane [Ruff and Lustig, Inorg. Synth., 1968, 11, 138–43]. The disadvantage of this process lies in particular in the high cost of AsF3, in its toxicity and in the risk of contaminating the compound obtained.
For the phosphoryl derivatives, a process for the preparation of LiN(POF2)2 consists in reacting LiN(SiMe3)2 with POF3. The removal of volatile Me3SiF results directly in the expected product [Fluck and Beuerle, Z. Anorg. Allg. Chem., 412(1), 65–70, 1975]. The disadvantage of this process lies in the cost of the silylated derivative and the use of gaseous and toxic POF3.
It is known to prepare a fluorinated compound from the corresponding halogenated compound by a halogen exchange reaction using an ionic halide, such as, for example, KF or CsF, or an organic fluoride, such as tetra(n-butyl)ammonium fluoride. The reaction is a nucleophilic substitution which preferably takes place in a polar aprotic solvent. The exchange reaction is promoted by the presence of a phase transfer catalyst chosen, for example, from quaternary ammonium salts, crown ethers, pyridinium salts or quaternary phosphonium salts. This process has been carried out with KF in particular to obtain monofluoroalkanes, α-fluoroesters, fluoroethers, acyl fluorides or sulfonyl fluorides respectively from the corresponding monohaloalkanes, α-haloesters, haloethers, acyl halides or sulfonyl halides [A. Basbour et al. in M. Stacy and co-editors, Advances in Fluorine Chemistry, Vol. 3, Butterworth, Washington D.C., 1963, pp. 181–250].